Seismic exploration systems employing arrays of detectors deployed vertically in well boreholes have become important in recent years. Usually such systems have a plurality of detectors deployed along a well borehole at different vertical depths. Each detector may comprise, for example, a one component hydrophone or a three component geophone (i.e. a geophone detector capable of detecting the x, y, and z orthogonal spatial components of seismic waves arriving at its location). These systems include an electronic digitizer for digitizing the measured geophone components' amplitudes, and also, a transmission system for transmitting the digitized signal representation as time series of digital numbers uphole to a receiving system including a recorder and processor. The vertically deployed seismic detectors are usually connected together by well logging cable, which may be of the conventional armored seven conductor type, or by fiber optic cable, if desired.
In conducting borehole seismic surveys it is desirable to deploy a large number of seismic detectors in a well. Usually modules as described above, may be spaced at 10 foot intervals and may be joined together by jumper cables. In using three component borehole seismic detectors it is important that each detector be firmly attached to the borehole wall in order to properly capture the three spatial components of particle motion of arriving seismic waves or signals. In the prior art this has typically been accomplished by the use of a mechanical locking arm which can be extended from the side of a tool to urge, or force, the body member of the tool against the borehole wall opposite the locking arm. These locking arm systems have usually been operated from the earth's surface by controlling electrical motors and gear drives attached to them, or by controlling the flow of hydraulic fluid to a down hole hydraulic drive system connected to the arm. Such locking arm mechanisms contain many precision moving parts and high pressure seals and rapidly become the most complicated part of the downhole seismic system, as well as that associated with most tool malfunctions or breakdowns. In this regard, a catastrophic tool breakdown of the locking arm system can cause the tool to become stuck in well borehole, requiring an expensive "fishing" operation to retrieve it.
Accordingly, it would be most desirable to have a borehole seismic tool locking system which is less complicated, more reliable, and less likely of catastrophic failure requiring fishing.
Other than catastrophic sticking, other modes of locking arm failure can also occur. Detector tool body members are generally of elongated cylindrical shape. Typically there are two pairs of stabilizing feet on the side of the tool opposite the locking arm (backside of the tool). One pair of feet is generally near the top of the tool and other pair of feet are near the bottom of the cylindrical tool module. The purpose of these feet is to prevent the tool from rocking or moving sideways and introducing undesirable oscillations into the measurements. If the tool is properly centered in the casing of a cased well borehole, this works well, as the locking arm works along a true diagonal of the casing. However, errors can occur because most well boreholes are not perfectly vertical. The elongated cylindrical tool body tends to ride against the downhole side of the casing as the tool is moved through the borehole. This results in one of the upper, lower, or both pairs of feet being in engagement with the borehole wall prior to deployment of the locking arm. When the locking arm is then extended, these contacting or engaged feet tend to falsely anchor the tool body to the downhole side of the casing, and only along that side. This provides an unstable, false anchor which may later release, allowing the tool to oscillate.
In the past it has been attempted to deploy the locking arms of the seismic tool modules while keeping the module moving to avoid this problem. This works well if only one tool module is used, but becomes increasingly difficult as more tool modules are employed. It would therefore, be highly desirable to provide a seismic tool locking system which could securely lock a tool in place while the tool was motionless and still avoid the tool "false locking" or eccentering problem just described.